The present invention relates to a device for optical distance measurement and to a method for optical distance measurement by means of phase modulation.
Optical distance measuring devices as such have been known for a long time and have in the meantime also been commercially marketed in a wide variety of embodiment forms. These devices emit a modulated light beam, which is oriented toward the surface of a target object to be measured, whose distance from the device is to be determined. Part of the light reflected against or scattered back by the target surface is detected again by the measuring device and used to determine the desired distance.
The application field of such distance measuring devices generally includes distances in a range of typically between a few centimeters and several hundred meters.
Depending on the distances to be measured and the reflectivity of the target object, different demands are placed on the light source, the quality of the measurement beam, and the detector.
Depending on the orientation of the transmission and reception conduits necessarily provided in the device, the optical distance measuring devices known from the prior art can essentially be divided into two categories. On the one hand, there are devices in which the transmission conduit is situated a certain distance apart from the reception conduit so that the respective optical axes are parallel to each other. On the other hand, there are monoaxial measuring devices in which the reception conduit is coaxial to the transmission conduit.
The biaxial measurement systems have the advantage of not requiring a complex beam splitting to select out the returning measurement signal, thus permitting better suppression of an optical crosstalk, for example, from the transmission conduit directly into the reception conduit.
On the other hand, biaxial distance measuring devices have among other things, the disadvantage that in the range of shorter measurement distances, detection problems can arise due to the presence of a parallax. The projection of the target object onto the detector surface of the measurement receiver integrated into the unit, which projection is still unambiguously situated on the detector at large target distances, begins to creep away from the optical axis of the reception branch as the measurement distance decreases, and also experiences a variation in the beam cross section in the detector plane.
This means that without further steps being taken in the unit, in the close detection range, i.e. for a short distance between the target object and the measuring device, the measurement signal can approach zero.
DE4316348A1 has disclosed a device for distance measurement, having a visible measurement beam generated by a semiconductor laser whose reception device includes a light guide whose outlet is connected to an optoelectronic converter. The light entry surface into the fiber of the light guide is situated in the projection plane of the reception lens of this device for large object distances and can be shifted out of this position laterally in relation to the optical axis.
In this way, measurement beams that arrive into the reception lens at ever greater inclination at short object distances can be guided by the device according to DE4316348A1 onto the light-sensitive surface of the detector through tracking by the optical fibers, without requiring spatial movement of the detector.
The required electronic control of the tracking and the use of additional, in particular moving parts in the distance measuring device according to DE4316348A1 entail a not insignificant amount of effort, which increases the complexity and therefore the cost and susceptibility of such a system to malfunction.
There are also known optoelectronic distance sensors that function in accordance with the so-called triangulation principle. DE3703422A1 has disclosed such an optoelectronic distance sensor functioning according to the triangulation principle, which has at least one first pilot beam source 7 that makes the projection beam path of the sensor visible, which is inclined in relation to the measurement beam. In sensors of this kind, a position-sensitive detector is used, which is offset in relation to the transmission direction. Since the incidence point of the measurement beam reflected by the target object is a function of the distance of the detector from the target object, its position can be used to calculate the distance between the detector and the target object.
DE29615514U1 has disclosed an electronic distance measuring device that has a flat measuring bar, which is situated on the side of the housing of the measuring device and can be slid in relation to it. The flat measuring bar serves to establish a reference plane spaced a definite distance apart from the measurement plane of the distance measuring device; the measuring device is then able to detect small distances very precisely by measuring a measurement value greater than the minimum and then subtracting the distance between these two planes to determine the measurement value. In a suitable fashion, the subtraction of this distance between the planes occurs automatically in that the degree to which the flat measuring bar extends out from the measuring device housing is measured at the push of a button and can be automatically taken into account. In this way, the measuring device according to DE29615514U1 is able to determine very small distances with a high degree of precision, even when using the phase-comparison method to determine the desired distance.
Triangulation sensors of this kind are typically used in industrial sensors for distance measurement, for example in machine tools to determine short movement distances of a moving part of such a machine tool. The triangulation measurement method only permits measurements in a small measurement range, but is able to achieve high levels of precision.